1. Field of the Invention
The present invention relates to an antenna device suitably used in a radio communication apparatus such as a cellular phone, or a radio communication apparatus such as a WLAN (wireless local area network) or an RFID (radio frequency identification).
2. Description of the Related Art
In recent mobile communication systems, various services other than a telephone call have been under study such as data communication or TV telephone using web browsing, a position information detection service using a GPS (global positioning system), or authentication and accounting using the RFID (radio frequency identification).
With the above study, the radio communication apparatus such as the cellular phone has been demanded to have a large number of functions in order to deal with various services. In addition, the radio communication apparatus has been demanded to stabilize the communication quality irrespective of the use state of the radio communication apparatus.
In general, the radio communication apparatus of the mobile type always changes its direction or inclination with respect to a communication party (base station) depending on its use state. It is assumed that, as a use state, a call in a state where the radio communication apparatus is made close to his head, or the user holds the radio communication apparatus apart from his head to conduct data communication other than a call. Even in the radio communication apparatus of the mobile type which always changes the use state according to the contents of the service, an antenna device that is stabilized in communication quality, particularly reception sensitivity has been demanded.
In general, the reception sensitivity of the antenna changes according to the direction or inclination with respect to the base station, which does not apply to the radio communication apparatus of the mobile type alone. As one countermeasure for preventing the deterioration of the reception sensitivity, there has been known an antenna diversity technique that uses a plurality of antenna elements, and selects the antenna element that is the highest in the reception sensitivity and receives communication data. However, because the plurality of antenna elements is required to be mounted, the antenna diversity technique is improper for the radio communication apparatus of the mobile type to be downsized.
Also, the radio communication apparatus deteriorates the reception sensitivity even due to the absorption of electric waves into an approaching human body. As a countermeasure for preventing the deterioration of the reception sensitivity, there has been known a method of controlling the directivity (radiating direction of the electric waves) of the antenna. As an example of controlling the directivity of the antenna, there is an array antenna technique that uses a plurality of antenna elements and synthesizes the electric waves that are radiated from the respective antenna elements by feeding signals different in phase and amplitude to the respective antenna elements. The array antenna technique is improper for the radio communication apparatus of the mobile type to be downsized because the antenna elements need to be arranged at given intervals, which leads to a large antenna device.
Also, there has been disclosed a technique of producing an arbitrary directivity by adding a reactance variable element or circuit to each of a plurality of non-electricity-feed antennas which are arranged in a circle about a electricity-feed antenna (Roger F. Harrington, “reactive controlled directive arrays”, IEEE transactions on antennas and propagation, vol. AP26, No. 3, May 1978, p390 to 395). In the technique, electric lengths of the non-electricity-feed antenna elements are so changed as to produce the arbitrary antenna directivity mainly on the horizontal plane (the same polarization plane). Also, an ESPAR (electronically steerable passive array radiator) antenna using the above principle has been disclosed in JP 2001-024431 A. In those techniques, because an electrical signal is fed to only one antenna element, a signal processor circuit is simplified more than the above array antenna to suppress an increase in power consumption. However, in order to change the directivity in a range of practical use, it is necessary to provide about 4 to 6 non-electricity-feed antenna elements, and therefore the above techniques are improper for the radio communication apparatus of the mobile type to be downsized.
Also, as an example of controlling the directivity of the antenna, Japanese Patent No. 3399545 discloses an antenna device that is made up of one electricity-feed antenna element and one non-electricity-feed antenna element. The antenna device suffers from such a problem that the controllable directivity pattern is limited.
In addition, JP2001-326514A discloses an antenna device in which the termination of a loop antenna is changed over between two states of short-circuited state and open state to change the directivity (vertical polarization or horizontal polarization). The antenna device can select the directivity according to the use state of the radio communication apparatus since the polarization plane can be controlled. However, the controllable directivity is limited to two directions. Also, it is necessary to provide an antenna element having a length as long as one wavelength of the frequency to be used because the loop antenna is used. Therefore, the entire antenna device is relatively large in size, and it is difficult to incorporate the antenna device into the radio communication apparatus of the mobile type.
The conventional antenna devices as described above suffers from such problems that the directivity of the antenna is limited, and the number of antenna elements is increased, or the antenna per se becomes large in size.